This invention relates to optical radar and more particularly to transceiver control apparatus which may be part of an airborne optical obstacle avoidance or target tracking radar set. Such radar sets are used in helicopters and other low flying aircraft to detect obstacles or track targets in the aircraft's flight path by transmitting forward of the craft a laser-generated coherent beam in the infra red region and detecting the echo signals from obstacles such as power lines, hills, trees and buildings.
Most optical radars of these types are of the heterodyne type in which the received optical signals are heterodyned in a mixer with the output of a laser local oscillator which has a fixed and controlled frequency which is offset from the laser transmitter's frequency. This results in an intermediate frequency signal in the output of the mixer which is equal to the aforementioned offset frequency. Target information such as range, target radial velocities and Doppler signatures of moving targets is obtained from the modulated intermediate frequency signals.
Heterodyne optical radars, especially those of the Doppler type, require laser transmitters and local oscillators which are highly stable in frequency and are controlled to operate at an accurate frequency offset so that the radar set's intermediate frequency is constant. Any frequency drift between the two laser frequencies will have the same affect on the intermediate frequency signal thereof as will radial target movement. Further, the high powered CO.sub.2 transmitter lasers required for such radar sets necessarily involve moderate to large Fresnel number optical cavities which have inherently low temporal and modal stability. The temporal instability arises when the differential optical loss among competing high order transverse and longitudinal modes is low, resulting in random "mode hopping" and consequent random output frequency changes. Moreover, without some form of optical dispersion, a high power CO.sub.2 l laser transmitter can oscillate on any number of vibrational-rotational transitions in the 9 to 11 micron spectral region, and while gratings or prisms may be employed to provide intracavity optical dispersion, these elements invariably add considerable optical loss.
Such inherently unstable large CO.sub.2 lasers can be stabilized or controlled by injecting into the cavity thereof a small sample of the desired frequency, wavelength and mode of operation, as long as the high power laser cavity has the required optical design to support this frequency or wavelength of oscillation. Under these conditions, the injected signal will force the higher powered device to operate on the injected transition and transverse mode. The source of the injection signal is usually another smaller CO.sub.2 laser, which has better temporal, modal and frequency stability due to its smaller cavity, and which can in addition be provided with an accurate frequency stabilization system, such as a feedback loop containing a Stark cell as a frequency reference.
A co-pending application entitled INJECTION CONTROLLED LASER TRANSMITTER WITH TWIN LOCAL OSCILLATORS, Ser. No. 704,816, filed on Feb. 25, 1985, now U.S. Pat. No. 4,655,588, issued Apr. 7, 1987, discloses and claims a heterodyne CO.sub.2 optical Doppler radar comprising a ring type of transmitter laser with twin local oscillator lasers which are automatically controlled to operate at a frequency difference equal to the desired intermediate frequency of the optical radar. The output of one of the two lasers is injected onto the transmitter laser for stabilization purposes and the output of the other of the twin lasers is heterodyned with the received target return signals to form the radar's intermediate frequency signal.
The present invention comprises apparatus and circuitry which achieves stable operation of a high power pulsed laser transmitter and an accurately controlled intermediate frequency signal with the use of only a single local oscillator laser.